1. Field of the Invention
The present invention relates to a method for sealing porous metals.
2. Description of Prior Art
Ordinarily, a sintered metal contains pores in an amount of 5 to 30% by volume. This sintered metal having pores impregnated with a lubricating oil is utilized as a lubricatless bearing. Such sintered metals are very often plated in order to improve the corrosion resistance or abrasion resistance, or for a decorative purpose. When such a plating is carried out, plating solution permeates into pores in the plating step and such plating solution cannot sufficiently be removed even if washing is performed after the plating. As the results, the metals are corroded at the peripheries of pores to reduce the corrosion resistance. In case of an iron type sintered alloy, carburization, carbonitriding or soft-nitriding is very often carried out so as to improve the abrasion resistance or strength. This treatment, for example, carburizing quenching, should be effected in a depth of about 1 mm from the surface to harden the surface, and it is necessary to retain the interior toughness. If the treatment such as carburization is performed on a porous metal member without effecting a sealing treatment in advance, however, hardening is advanced to a depth exceeding 1 mm, with the result that the interior toughness is drastically reduced and the metal member cannot be put into practical use.
Therefore, sealing treatments are usually carried out before such treatments as plating, carburization, carbonitriding and soft-nitriding are conducted. Examples of such sealing treatments are disclosed by Japanese Patent Application Laid-Open Specification No. 56-35704 and Japanese Publication "Metal Surface Technique", Vol. 32, No. 8, 1981, published by the Metal Surface Technique Association. The sealing treatments are carried out by one of the following steps.
(1) A porous metal member is impregnated with paraffin or a polymeric material. PA1 (2) A porous metal member is impregnated with a water glass material. PA1 (3) The topmost surface of a porous metal member is subjected to a mechanical plastic processing to effect sealing.
The method (1) is carried out as a preliminary treatment for the plating operation. In this method, paraffin or a polymeric material is stuck even to a portion other than surface pores, that is, a substrate portion, and therefore, a step of removing the paraffin or polymeric material becomes necessary. Furthermore, the method (1) is defective in that since the difference of the thermal expansion coefficient between the material to be treated and the impregnation treatment agent is large, complete sealing is hardly attained.
The method (2) is utilized as a preliminary treatment when carburization, carbonitriding or soft-nitriding is performed in the heated state. The method (2), however, is defective in that the impregnation material is left after the sealing treatment and the characteristics of the porous member, such as the light weight and porosity, are degraded.
According to the method (3), the plastic flow is caused on the surface of a porous material by a roll or coining mold to crush pores. However, the shape of the material to be treated is limited to a specific shape and only a material in which the plastic flow is readily caused can be treated. Accordingly, this method is not a general method.
As another method, there has been known a method in which an oxide film is formed on inner surfaces of effective pores extended to the interior by an oxidation treatment to effect sealing. In this method, however, since even the substrate on the topmost surface is oxidized to form an oxide film, a preliminary treatment such as a shot blasting treatment or a barrel treatment becomes further necessary for removing this oxide film.
Moreover, there is known a method in which an iron type sintered alloy is impregnated with copper or a copper alloy to improve the strength. There also is known a method for improving the abrasion resistance of a sintered alloy to be used as a valve seat of an engine by impregnating the sintered alloy under a pressure with a low-melting-point metal or alloy such as lead or a lead alloy by utilizing an autoclave or the like.
It is apparent that when a sealing treatment is carried out as a preliminary treatment for carburization, carbnitriding or soft-nitriding of a porous metal member such as an iron type sintered alloy, it is sufficient if only effective pores present in the vicinity of the topmost surface of the porous metal member are sealed. However, if the sealing treatment is performed by the above-mentioned melt impregnation method or compression impregnation method, the treating agent such as a molten metal or alloy does not remain only in the portion close to the surface but permeates into the deep portion through pores, and therefore, it is necessary to use a large quantity of the treating agent. Furthermore, if the treating agent permeates even into the deep portion, the pores are closed and the characteristic property of the porous member, that is, the porosity, is degraded.